The invention relates to a measuring device as described below for measuring the flow velocity of a medium flowing through a measuring tube as well as to a method as described below for measuring the flow velocity of a medium flowing through a measuring tube.
Devices for magnetically-inductively measuring the flow velocity of a medium are known, for instance, from International patent application publications numbers WO 2008/107460 A1, WO 03/098164 A1 and WO 2007/031053 A2. They usually comprise means for generating a magnetic field perpendicular to the flow direction of the medium as well as electrodes arranged on a wall of a measuring tube in a plane perpendicular to the flow direction of the medium and via which electric voltage building up within the medium is detected. The measurement signal detected by means of the electrodes is then supplied to a measuring unit in which the measurement signal is evaluated.
Magnetic-inductive flow sensors (MID) utilize the fact that a medium flowing in a measuring tube penetrated by a magnetic field will induce an electric field which can be measured via the electrodes in the form of a voltage. Same is directly proportional to the flow velocity of the medium. Based on the flow velocity, the flux of the medium can then be calculated. A galvanic or capacitive signal pick-up can be used for signal decoupling in such measuring devices. Galvanic signal decoupling is mostly used, in which metallic electrodes of small diameters (in general, a few millimeters) equipped with feedthroughs through the inner tube wall are in direct (galvanic) contact with the medium. They need to be reliably sealed against the tube wall. This type of decoupling is characterized by a sturdy and simple construction, but is susceptible to chemical attack, deposits and abrasion of the electrodes and the seals. Controlling high operational temperatures requires careful coordination of the thermal expansion coefficients of the electrode and wall material.
In capacitive signal decoupling, the electrode is not in direct contact with the medium but is surrounded by an insulating layer which is in turn in contact with the medium.
Commercially available magnetic-inductive flow sensors work based on time-variable magnetic fields, so-called alternating fields, in order to eliminate interference signals superimposed on the usable signal and are differentiated according to rapidly changing noise and slowly changing drift components. The essential sources of these interference signals are time-variable, not precisely determinable double-layer potentials at the interface between the flow medium and the coupling element, i.e., in particular the electrode. To take these signal components into account, current-energized field coils are required which are mostly operated in a pulsed mode; i.e., clocked. A time-variable magnetic field of a known magnitude is required in both galvanic as well as capacitive decoupling of measurement signals, entailing additional energy expenditure in generating the magnetic fields. Such measuring devices with alternating fields also only allow a discontinuous measuring with the pulse repetition frequency of the magnetic field. The required pulse repetition frequency of the magnetic field furthermore depends on the frequency spectrum of the interference voltages. Therefore, there are efforts to develop measuring devices which do not require time-variable magnetic fields but make do with one permanent magnet while still enabling the suppression of interference signals.
Due to their considerable influence on measuring accuracy, approaches to create magnetic-inductive systems and components for signal decoupling in magnetic-inductive flow sensors are known which have considerably more favorable properties in terms of their noise and drift behavior than conventional measuring devices comprising purely metallic electrodes of a galvanic signal decoupling structure. Examples of this are described in U.S. Pat. No. 4,517,846 and International patent application publication number WO 98/55837 A1. Both propose coating metallic electrodes with very hard metal oxides such that the flow medium only comes into direct contact with this layer, the latter being of an electrically insulating yet porous structure. This porous structure can be produced in the layer manufacturing process itself or else by subsequent selective processing.
Such a porous layer has the advantage of a noise-reducing and drift-reducing effect defined by medium penetrating into the porous structure. The porous structure creates an enlarged contact surface between the medium flowing within the measuring tube and the solid body, hence the signal decoupling structure. Furthermore, ion channels form within the pores, providing conductivity to the intrinsically insulating layer. The layer is moreover attributed to the capacity of ion storage, which serves as a charge buffer and thus has a noise and drift-reducing effect.
More recent literature also reports on fractal surface structures of layers, e.g., of titanium nitride, which are used with great success in medical technology as noise and drift-reducing layers for signal coupling and decoupling—e.g., in cardiac pacemakers (see E. Wintermantel, “Medizintechnik”, Springer, Berlin, 5th edition, pp. 1338-1343 (2009)). Similar solutions can be expected to also be suitable for signal decoupling from a magnetic-inductive flow sensor and leading to a further suppression of the electrochemically induced noise, drift and step signals there.
It is further desirable to create a magnetic-inductive flow sensor which does not depend on a current flow in the medium. No-current measurement has decisive advantages. In particular, once charge balances have been adjusted, measurements will not disturb them since there is no associated substance transport whatsoever. This has an advantageous effect in terms of further reducing drift. Although most methods for signal decoupling in magnetic-inductive flow sensors require a current flow, a no-current approach is known from German Patent DE 10 2005 043 718 B3. This describes measuring via a purely electric field effect ensuing from the medium flowing in the measurement tube interacting with the magnetic field. The electric field has in this case a controlling effect on the resistance of an adjacent semiconductor. In a specific configuration here, a pair of ISFETs (ion-sensitive field-effect transistors) takes over the signal decoupling, wherein the transistor gate electrodes provided with an insulating layer are each in contact with the flowing medium. In the simplest case, the insulating layer on the gate electrode consists of silicone dioxide. This material is known for being able to store and release H+ ions from its surface (P. Bergveld, “ISFET, Theory and Practice,” IEEE Sensor Conference, Toronto; pp. 1-26 (October 2003)), thereby resulting in a balance of the surface charge contingent upon the pH value of the flowing medium and a double layer voltage induced by said surface charge. The surface of the insulating substance acts like a charge accumulator which stabilizes the double layer voltage due to its buffer action and thus contributes effectively to noise suppression. The supply voltage—superimposed by the difference in the double layer voltage—is measured between the gates of the ISFET pair while the ISFET path resistances are controlled by same.
As stated above, no-current measurement particularly has the decisive advantage that once set, charge balances will not be disturbed by a measurement since there is no substance transport whatsoever associated with the measurement. In practice, however, every insulating substance has a low but finite conductivity so that a no-current measurement is in reality only approximated. The described ISFET system for signal decoupling can therefore also be understood as a combination of a pair of ion storage layers and a differential amplifier having an extremely high input resistance.
The measuring devices for magnetic-inductive measuring described above and known from the prior art have the disadvantage of separately optimizing the measuring device with regard to minimizing noise and interference signals as well as not being possible to realize virtually no-current measurement.
It is therefore the task of the present invention to create a measuring device and a method for measuring the flow velocity of a medium flowing through a measuring tube which combines the advantages of virtually no-current field measurement with the advantages of signal decoupling via non-metallic layers and which can be operated with permanent magnets.
A measuring device having the characterizing features described below is proposed to solve this task. Advantageous configurations of the measuring device are also described below.